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Development and Validation of Residual Stress Test for Concrete Pavement. David Marks, Daniel Castaneda and David A. Lange University of Illinois at Urbana-Champaign FAA Worldwide Airport Technology Transfer Conference Atlantic City, New Jersey April 2010. Acknowledgments.
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Development and Validation of Residual Stress Test for Concrete Pavement David Marks, Daniel Castaneda and David A. Lange University of Illinois at Urbana-Champaign FAA Worldwide Airport Technology Transfer Conference Atlantic City, New Jersey April 2010
Acknowledgments • Sponsored by FAA through Center of Excellence for Airport Technology • Idea for this new test originated with Edward Guo • David Marks finished his MS at UIUC • Daniel Castaneda is currently extending this work for his MS at UIUC
Introduction • What are residual stresses? • Residual stress may diminish capacity to sustain designed load • Problem: No standard method to measure residual stress in concrete • Inspiration: Residual stress test method for steel • ASTM E837-08 Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method • A strain gage measures the change in strain reading as a small hole (~2mm) is drilled in the vicinity. The change in strain reading is correlated to a residual stress
Overview of test • FAA NAPTF adapted ASTM E837 to cantilevered concrete beams with encouraging results • • Load applied at one end of concrete beam • • Core drilled adjacent to mounted strain gage • • Change in strain corresponds to relieved stress SIDE VIEW TOP VIEW
Validation Testing by UIUC Similar setup to NAPTF: Cantilevered concrete beam with 430 lb (1.94 kN) applied load Added end deflection measurement 20mm and 30mm strain gages covered with an inert sealant and aluminum tape to protect from core-drill cooling water While loaded, a 3” (7.62cm) diameter hole is cored atop the concrete beam at ¼” (0.635cm) intervals to a total depth of 1.5” (3.2cm)
Validation Testing by UIUC Core drill
Validation Testing by UIUC • Practical problems: • Water from drilling is problematic • Heat from drilling is problematic…required 10 min cool time • How deep? Increased isolation of the strain vicinity is observed as core-depths increase…to a point. • Even unloaded beams have some residual stress! (due to drying stresses)
Validation Testing by UIUC 0.6cm cut 1.3cm cut 2.5cm cut 3.2cm cut 1.9cm cut
Validation Testing by UIUC Strain reading progressively drops from 40με to 8με for a strain change of 32με. This correlates to a stress reduction of approximately 190psi. The applied load produces a theoretical tensile stress of 230 psi at the location of the strain gage.
Validation Testing by UIUC The load-strain response of the concrete beam is diminished as the depth of the core is increased, suggesting partial isolation of the applied load.
So, a new approach…sawcut • Testing was modified by replacing the core-drill with a circular saw fitted with a masonry blade and cutting linear notches on either side of the strain gage • Concrete beams were either singly notched or doubly notched • Progressive depths of 1.27cm, 2.54cm and 3.56cm • Wooden spacers placed over the strain gage to serve as guide and to prevent contact with underside of circular saw
Modified Testing by UIUC Core drill Linear Notches
Modified Testing by UIUC Singly notching the concrete beam on either side produced similar results to core-drilling Time of strain recovery lessened possibly due to quick pass of circular saw (~30sec) generating less heat Doubly notched concrete beams could achieve full strain relaxation
Modified Testing by UIUC 0.64cm 1st cut 1.27cm 1s cut 2.54cm 1st cut 3.56cm 2nd cut 3.56cm 1st cut 1.27cm 2nd cut 0.64cm 2nd cut
Modified Testing by UIUC The first notch progressively decreases the strain reading from 25με to -25με for a strain change of 50με. This correlates to a stress reduction of approximately 340psi.
Modified Testing by UIUC The second notch progressively decreases the strain reading from -25με to -40με for an additional strain change of 15με. This correlates to a stress reduction of approximately 102psi.
Modified Testing by UIUC In combination, the strain reading decreases from 25με to -40με for a strain change of 65με. This correlates to a stress reduction of approximately 440psi. With a theoretical applied stress of 230psi, the estimated residual stress in an unloaded beam is 210psi.
Modified Testing by UIUC As the load is removed and applied, there is no response in the concrete beam suggesting that full isolation has been accomplished.
Validation Testing by UIUC The load-strain response of the concrete beam is diminished as the depth of the notch is increased. For a doubly notched beam, a flat, non-responsive line is observed indicating full relaxation of stresses.
FEA of Notches in Concrete Beams Plain model (with no notches) was made as a reference Singly notched beams were modeled with depths of 1.27cm, 2.54cm and 3.81cm Doubly notched beams were spaced 8.89cm apart and modeled with paired depths of 0.635cm, 1.27cm, 2.54cm and 3.81cm MatLAB script averages the stress readings over a 30mm length and converts to a normalized strain reading for comparison
FEA of Notches in Concrete Beams • Good agreement between theoretical strain values and experiment strain values • Full relaxation of strains occurs when… • Higher ratios give worse results with small compressive stresses created in the strain gage area
Small compressive stress occurs if notch is too deep Recommended notch/space ratio
Conclusions • “An elegant solution” • Measuring residual stress by saw notching is effective and simple to implement • Full-scale slab tests are encouraging • Goal is to develop new ASTM test method